Field of the Invention
The present invention relates to a composite separation film including a cellulose fiber and a silica, and a method for manufacturing the same.
Background Art
A lithium secondary battery receives attention as a main mobile energy source of a ubiquitous era, and is a very promising field where application thereof from an existing power source for IT equipment, such as a cellular phone, a PDA, and a notebook computer, to a current mass capacity high power field, such as power tools, hybrid electric vehicles (HEV)/plug-in hybrid electric vehicles (PHEV), and energy storage systems, are rapidly expanding, and a market scale thereof is expected to reach about 21 trillion won by about 2020.
According to an increase in capacity of the lithium secondary battery and continuous expansion of application to the mass capacity high power field such as power tools, robots, and electric vehicles, importance regarding safety of the battery, such as explosion or ignition of the battery, has received large attention. It can be said that an issue regarding safety of the battery is, while considering that the lithium secondary battery is an important future energy source of the ubiquitous era, a core matter that should be surely solved for continuous development of the battery.
Up to now, in the lithium secondary battery, a polyolefin-based material has been used as a separation film material, but due to a material characteristic and a characteristic of a manufacturing process including stretching, severe thermal shrinkage is exhibited at a temperature of 100° C. or more, and a drawback where the lithium secondary battery is physically and easily broken by internal impurities of the battery, such as metal particles, is exhibited, and thus the polyolefin-based material is understood as an ultimate cause incurring an internal short circuit of the battery.
Meanwhile, since the polyolefin-based separation film shows hydrophobicity due to the material characteristic, the polyolefin-based separation film has low affinity with an electrolyte that is polar, and thus it is difficult for the electrolyte to penetrate the polyolefin-based separation film, which may cause a possibility of liquid leakage to the outside of the battery, and an impregnation property of electrolyte is low to negatively affect final performance of the battery. This phenomenon is more remarkably exhibited in an applied field requiring high power, such as a battery for an electric vehicle.
Therefore, there is an earnest demand for development of a next-generation separation film where thermal/mechanical stability, affinity to the electrolyte, and the like, which are basic drawbacks of the existing polyolefin-based separation film, are improved, and research thereof may become an epoch-making plan advancing development of a lithium secondary battery with high stability and performance.